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1. Field of the Invention
This invention pertains generally to methods and devices for sensing angular positions and, more particularly, to methods and devices for detecting and providing feedback of the angular position of a rotary control valve.
2. Description of the Prior Art
Rotary control valves, such as butterfly valves or ball valves, include a valve body and a plate, ball, or other flow control member rotatably mounted in the valve body to either block fluid flow through the valve, or allow fluid flow through the valve, depending upon the angular position of the flow control member. For example, a ball valve includes a ball which is securely mounted to upper and lower portions of a valve shaft. The ball is mounted in the fluid flow path of the valve by mounting the lower portion of the valve shaft in a lower portion of the valve body and the upper portion of the valve shaft in an upper portion of the valve body, with the ball positioned between the upper and lower shaft portions in the fluid flow path of the valve. An actuator is attached to the upper portion of the valve shaft, which extends through the valve body. When the actuator is turned, the valve shaft, and, therefore, the valve ball, is rotated. The ball is shaped, i.e., portions of the ball are removed or grooves are formed therein, such that when the ball is rotated, through, e.g., 90xc2x0, the fluid flow path through the valve is gradually opened or closed.
It is often desirable to determine accurately the angular position of the ball within the ball valve, and therefore, the state, i.e., open, closed, or somewhere in between, of the valve. Several methods of automatically determining the angular position of a valve are known. Sensing the angular position of the valve may be accomplished by attaching an angular position sensor to the valve actuator. For example, magnets may be attached to the rotating member of the valve actuator, and a Hall effect sensor used to determine the position of the actuator as the actuator member, therefore, the magnets attached thereto, is rotated. As the actuator member is rotated, the magnetic field produced by the magnets attached to the actuator is also rotated. The Hall sensor is placed within the magnetic field produced by the magnets. As the direction of the magnetic field changes, as the actuator is rotated, the Hall effect sensor detects the change and provides a signal from which the rotary position of the actuator can be determined.
Alternatively, a cam may be attached to the actuator shaft. The angular position of the actuator shaft is then converted to an electrical signal by an inductive sensor connected or in close proximity to the cam. As the actuator is rotated, the cam attached thereto is also rotated, which, in turn, changes the inductance of the inductive sensor in contact with or in close proximity to the cam. Thus, a signal is provided by the inductive sensor which is related to the angular position of the actuator and from which the angular position of the actuator can be determined.
As a third alternative, a potentiometer may be connected to the rotating member of the valve actuator. As the actuator member is rotated, the potentiometer potential is changed. This change in potential can be detected and signal derived therefrom from which the angular position of the actuator can be determined.
All known methods for determining the angular position of a valve by mounting a rotary position sensor on the valve actuator, however, suffers from a serious limitation. For a ball valve, for example, an accurate determination of the rotary position of the valve ball is desired. Although the valve actuator is connected, via the valve shaft, to the valve ball, there could be some inherent looseness in this connection. Even if the connection between the valve actuator and the ball is initially tight, this connection can fail or become looser with time. Thus, sensing the angular position of the valve actuator will not necessarily translate into an accurate indication of the position of the valve flow control member.
In order to determine the angular position of the flow control member more accurately, the angular position sensing methods described above have been employed to sense the position of the lower portion of the valve shaft which is directly connected to the flow control member. Since the shaft is directly and tightly connected to the flow control member, sensing the angular position of the shaft will result in an accurate determination of the angular position of the flow control member itself. Any of the angular position sensing methods described above may be used to determine the angular position of the valve shaft. For example, a magnet may be attached to the lower portion of the valve shaft, and a Hall sensor placed near the magnet. As the valve shaft, and, therefore, the flow control member itself, rotates, the magnetic field produced by the magnet attached to the valve shaft changes direction. This change in direction is detected by the Hall effect sensor, which provides a signal related to the angular position of the angular shaft member from which the angular position of the flow control member can be determined accurately.
Alternatively, a potentiometer can be attached to the lower portion of the valve shaft. As the shaft, and, therefore, the flow control member itself, is rotated, the potential of the potentiometer is changed. This change can be sensed, and a signal provided from which the angular position of the flow control member can be determined accurately.
Although measuring the angular position of a flow control member by sensing the angular position of the lower portion of the valve shaft can achieve accurate results, known methods for making such measurements suffer from other limitations. In order to measure the movement of the lower portion of the valve shaft, with a potentiometer or another device, the lower portion of the valve shaft must be extended through the bottom of the valve, and the potentiometer or other measurement device attached to the shaft on the outside of the valve. Extending the lower portion of the valve shaft thus provides another leak path from the valve, and the added packing adds friction to the valve Also, extending the lower portion of the shaft through the valve body makes the valve more fragile during moving and handling of the valve.
As an alternative to extending the lower portion of the valve shaft through the valve body, the potentiometer or other device for sensing angular position of the shaft may be extended through an aperture in the valve body near the end of the shaft. For example, a potentiometer may be mounted on the outside of the valve body. An elongated shaft attached to the potentiometer may be extended through an aperture in the valve body wall and be connected to the lower portion of the valve shaft. Alternatively, a Hall effect device may be mounted within the valve body, near a magnet placed on the flow control member or lower portion of the valve shaft, with conducting wires for conducting the signal provided by the Hall effect sensor passing through a hole in the valve body. In either case, the addition of another aperture to the valve body provides another potential leak path from the valve, and therefore, adversely affects valve integrity.
Another limitation of Hall effect and other magnetic field sensors employed to detect the angular position of the lower shaft of a rotary control valve is the effect of temperature changes on the accuracy of such devices. Changes in temperature of the magnet mounted on the lower portion of the valve shaft and the magnetic field sensing device itself can affect the signal provided by the sensor. Temperature changes, can, therefore, affect the accuracy of the angular position sensed by such a detector unless temperature compensation is provided.
The present invention provides for accurate detection of the angular position of a valve flow control member in a rotary control valve using magnets mounted in the bottom end of the lower portion of a valve shaft, which is tightly connected to the flow control member, and a magnetic field sensor, mounted outside of the valve pressure boundary, for detecting changes in the magnetic field produced by the magnets as the valve flow control member is rotated. Since, in this manner, the angular position of the lower valve shaft is determined directly, and since the lower valve shaft is tightly connected to the flow control member, the present invention provides a highly accurate determination of the angular position of the valve flow control member. Furthermore, since angular position detection in accordance with the present invention employs magnets which are mounted entirely within the valve, and a magnetic field sensor which is mounted entirely outside of the valve, the present invention allows accurate angular position detection to be achieved without the need for providing another hole through the valve, which would add another leak path from the valve, require additional packing, and make the valve more fragile.
In accordance with a first embodiment of the present invention a rotary control valve includes a ball, disk, or other flow control member, which is tightly connected to a valve shaft. The lower portion of the valve shaft is mounted on the inside of a lower portion of the valve wall. In accordance with the present invention, the lower portion of the valve shaft is made of a non-magnetic material, and has two magnets retained in cavities formed on each side of and extending parallel to the axis of rotation of the valve shaft. The magnets are oriented such that the north pole of one of the magnets and the south pole of the other magnet are near the bottom end of the valve shaft. A plate of ferrous material may be used to connect the other, upper, ends of the magnets through an opening in the valve shaft which extends between the cavities in which the magnets are retained. The purpose of the plate of ferrous material is to increase the strength of the magnetic field created between the lower ends of the magnets near the bottom of the lower portion of the valve shaft. An arching magnetic field is thus produced between the lower poles of the magnets at the bottom of the lower portion of the valve shaft. This arching magnetic field extends beyond the end of the valve shaft, and through the lower portion of the valve wall that is in close proximity to the end of the shaft. The lower portion of the valve wall penetrated by the magnetic field is made of a non-magnetic material.
A magnetic field sensor, such as a giant magneto resistive (GMR) sensor or a Hall effect sensor, is placed in the magnetic field created by the magnets on the outside, or unpressurized side, of the non-magnetic lower portion of the valve wall. The output signal provided by the magnetic field sensor is dependent on the strength and direction of the magnetic field in which the sensor is placed. As the lower portion of its valve shaft, and, therefore, the valve flow control member, is rotated, a sensor signal provided by the magnetic field sensor varies as the angular position of the magnets mounted in the lower portion of the valve shaft varies. Thus, the signal produced by the magnetic field sensor indicates the angular position of the valve flow control member. The sensor signal produced by the magnetic field sensor can be converted by a signal conditioner into an analog or digital signal format. This signal can be processed and transmitted to a position attached to or near the valve for accurate control of the valve flow control member position, and/or can be displayed at a local or remote location.
In an alternative embodiment of the present invention, the lower valve shaft, which is tightly connected to the valve flow control member, contains a cylindrical opening formed therein extending from the bottom of the shaft and centered on the axis of rotation of the shaft. Magnets are placed in two recesses formed in the shaft on opposite sides of the cylindrical opening. The magnets are placed in the recesses such that opposite poles point toward each other across the cylindrical opening to create a magnetic field within the cylindrical opening. A lower portion of the valve wall is formed to include an extension which extends into the cylindrical opening in the lower valve shaft. This lower portion of the valve wall is made of a non-magnetic material. A cavity is formed in the extending portion of this non-magnetic lower portion of the valve wall, on the outside of the valve wall, such that the magnetic field produced by the magnets in the valve shaft is also present within the cavity. A magnetic field sensor, such as a magneto-resistive sensor or a Hall effect sensor is placed in the magnetic field within the cavity. The magnetic field sensor produces a sensor signal which is dependent on the strength and direction of the magnetic field in which the sensor is mounted. Thus, as the lower portion of the valve shaft, and therefore, the flow control member, rotates, the sensor signal provided by the magnetic field sensor varies as the angular position of the magnets mounted in the valve shaft changes. Thus, the sensor signal provided by the magnetic field sensor provides an accurate indication of the angular position of the valve flow control member. The sensor signal can be converted by a signal conditioner into any analog or digital format, processed, and transmitted to a position attached to or near the valve to accurately control the position of the valve flow control member, and/or to the local or remote location for display.
The accuracy of rotary valve angular position detection in accordance with the present invention is improved by making the detection of the angular position of the valve flow control member insensitive to temperature changes in the magnets mounted in the lower valve shaft and the magnetic field strength sensor employed. This is achieved by using two magnetic field sensors mounted on the outside of the lower portion of the valve wall within the magnetic field produced by the magnets mounted in the lower valve shaft. The magnetic field sensors are mounted on the valve such that the active axes of the two sensors are oriented in the same plane but angularly displaced from each other. By combining the sensor signals provided by the two magnetic field sensors, the angular position of the valve flow control member can be calculated in a manner in which the first order dependence of position signal versus temperature is canceled out. Thus, in accordance with the present invention, the angular position of a flow control member in a rotary control valve can be determined under various temperature conditions.
Further objects, features and advantages of the present invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings.